1. Field of the Invention
The present invention relates to a device and method for controlling a secondary battery, wherein the device and method are used for an electronic device that uses, as an operation power supply, a secondary battery (sometimes referred to simply as “battery”, hereinafter) such as a lithium ion battery, and accurately detect the remaining capacity of the secondary battery when the electronic device enters a power-saving mode (sometimes referred to simply as “sleep mode”, hereinafter).
2. Description of the Background Art
Secondary batteries are popular as power supplies for supplying power to portable electronic devices such as mobile phones, digital cameras, and notebook PCs. The capacity of such a secondary battery, which is chargeable by single charging, is limited. In order to continuously use an electronic device having a secondary battery, it is necessary to charge the secondary battery before the remaining capacity thereof becomes zero (in other words, before the secondary battery is fully discharged). Therefore, the user must check the remaining capacity. The remaining capacity is displayed on a screen mounted on the electronic device. If the electronic device is a notebook PC, the performance of the notebook PC may be changed in accordance with the remaining capacity (for example, the brightness of the screen may be reduced), or shutdown may be performed before the remaining capacity becomes zero. In order to perform these processes, it is necessary to accurately calculate the remaining capacity of the secondary battery.
In the electronic devices (such as mobile phones, digital cameras, and notebook PCs) each having a secondary battery as a power supply, an operation mode called a power-saving mode (sleep mode or standby mode) is often utilized for the purpose of reducing power consumption. The power-saving mode is as follows. When an electronic device need not be driven (for example, when a notebook PC is not operated for a predetermined time or more), supply of power to some loads (including various functions and control microcomputers of the notebook PC, particularly, a power supply control microcomputer) is temporarily stopped to minimize power consumption. Thereby, the driving time period of the notebook PC by the secondary battery is maximized.
Even during the power-saving mode, minute current is discharged from the battery. Therefore, it is necessary to accurately calculate the remaining battery capacity even during the power-saving mode. Japanese Laid-Open Patent Publication 2009-183067 (Patent Literature 1) discloses a technique of readily measuring a discharging current value during the power-saving mode to accurately calculate the remaining battery capacity. Japanese Laid-Open Patent Publication No. 2004-198196 (Patent Literature 2) discloses a technique of accurately calculating the remaining battery capacity even when a discharging current value cannot be detected during the power-saving mode. Japanese Laid-Open Patent Publication No. 2010-19757 (Patent Literature 3) discloses a technique of accurately obtaining the remaining battery capacity by restoring the operation mode from the power-saving mode to the normal mode at predetermined time intervals.
However, Patent Literature 1 merely discloses the technique of previously storing the discharging current value in the power-saving mode, and calculating the remaining battery capacity by using the stored current value. In the technique disclosed in Patent Literature 2, since a remaining capacity calculating microcomputer calculates the remaining battery capacity by using the discharging current value (estimated value) also during the power-saving mode, it is necessary to supply power to the remaining capacity calculating microcomputer even during the power-saving mode. In the technique disclosed in Patent Literature 3, since the remaining battery capacity is detected by intermittently performing temporal restoration from the power-saving mode to the normal mode, the period of the power-saving mode is short and thereby the power-saving effect is reduced. Thus, the techniques disclosed in Patent Literatures 1 to 3 cannot accurately calculate the remaining battery capacity while taking advantage of the effect of the power-saving mode.
If the remaining battery capacity cannot be accurately calculated during the power-saving mode, the following problem may occur. In the case of a notebook PC, shutdown is performed before the PC becomes inoperable due to reduction in the remaining battery capacity during the power-saving mode. If this shutdown is performed depending on inaccurate remaining battery capacity, the remaining battery capacity might become 0 (zero) (in other words, the battery might be fully discharged) before the shutdown is normally completed. This situation might cause a serious problem such as data loss.
In the case of a notebook PC having two batteries, the following problem may occur. Such a notebook PC is provided with a switch for selecting either of the two batteries to supply power to loads. The switch is controlled by a power-supply control microcomputer, and the power-supply control microcomputer is not operated during the sleep mode. If the remaining capacity of one of the two batteries is reduced during the power-saving mode, the power-supply control microcomputer controls the switch to select the other battery before the switch becomes inoperable (in other words, the power-supply control microcomputer controls the switch to select the other battery while the power-supply control microcomputer can be driven by the one battery). If this switching is attempted to be performed based on an inaccurate remaining battery capacity, the remaining battery capacity might become 0 (zero) (in other words, the battery might be fully discharged) before the switching. In such case, since power cannot be supplied from the fully-discharged battery to the power-supply control microcomputer, the one battery cannot be switched to the other battery. Although the notebook PC has the two batteries and the other battery has a remaining capacity, battery switching cannot be performed.